Various instruments utilize conduits for transportation of process fluids and sample compounds and/or for separation of sample compounds. For example, chemical-analysis instruments that utilize liquid chromatography (LC), capillary electrophoresis (CE) or capillary electro-chromatography (CEC) perform separation of sample compounds as the sample passes through a column. Such instruments include conduits or have connections to conduits that transport a variety of materials, such as solvents and sample compounds.
In addition to tubing, including separation column(s), liquid-chromatography instruments typically include reservoirs, pumps, filters, check valves, sample-injection valves, and sample compound detectors. Typically, solvents are stored in reservoirs, and delivered as required via reciprocating-cylinder based pumps. Sample materials are often injected via syringe-type pumps.
In some cases, separation columns include one or more electrodes to permit application of a voltage to a sample-containing fluid passing through and/or exiting from the conduit. CEC, for example, utilizes an electro-osmotic flow (EOF) to propel a mobile phase through a chromatographic column. In contrast, liquid chromatography, such as high-performance liquid chromatography (HPLC), relies on pressure to propel a fluid through a column.
Suitable analytical-instrument tubing withstands pressures encountering during fabrication and use, is reliable through repeated use, and has physical and chemical compatibility with process and sample compounds. Generally, a tube material should not corrode or leach, and sample compounds should not adhere to the tube (unless required for a separation process.)
For HPLC and higher-pressure applications, tubing is typically made from stainless steel or fused silica to provide suitable strength and cleanliness. Such tubing is typically joined to other components via stainless steel connectors.
Stainless steel, however, has disadvantages in some applications due to its biocompatibility limits in comparison to some other materials; some organic molecules tend to adhere to the inner walls of steel tubing, and components of a steel alloy at times leach into fluid passing through the tubing. Because organic molecules generally are less likely to stick to glass than to steel, steel tubing can be lined with glass to improve biocompatibility, but such tubing is vulnerable to breakage.
For good biocompatibility, tubing can be fabricated from suitable polymeric materials. To compensate for relatively poor strength, some polymer tubes have relatively thick walls with a fluid lumen produced by machining and polishing. Such columns are typically costly to manufacture. Moreover, the lumen surface is unsuitable for some applications.
Typically, tubing must also be compatible with connectors that provide fluidic connections to other components of an instrument. Problems associated with the design and use of connector fittings are particularly difficult for high-pressure fabrication and operation. For example, pressures in the range of 1,000-5,000 pounds per square inch (psi) or higher are often utilized in liquid chromatography.